Stair-step cracks on an old facade in Nice: cause and severity

You’re looking at the wall of your old house in Nice, and you see that crack climbing diagonally, like a staircase. It wasn’t there two years ago. Honestly, that’s never trivial. In practice, a stair-step crack following the mortar joints is often a sign of differential movement between two parts of the wall. DTU 20.1 (NF P 10-203) states that facade masonry must withstand loads without structural disorder. As soon as you step outside that framework, you need to understand why.

Telling an active crack from a stabilised one

The first question: is it still moving? A stabilised crack is often old, a settling from the early years. An active crack is something else. Simple test: place a fine plaster tell-tale (or just a pencil line) across the crack. Write the date. Check every three months. If the line breaks, it’s still working. A crack gauge costs a few dozen euros and gives a measurement in millimetres. That’s the basis before any conclusion.

In Nice, old houses in the Vieux-Pays or hillside districts like Cimiez or Mont Boron are often founded on heterogeneous ground. BRGM has mapped clay-rich areas sensitive to shrink-swell in the Alpes-Maritimes. Soil that swells in winter and shrinks in summer works the foundations. And a facade follows.

The most common causes on a Nice house

There are three families of causes. First, foundation movement: clay shrink-swell, poor bearing capacity, or shallow footings on sloping ground. Second, water: a blocked rainwater downpipe, repeated infiltration, poorly directed runoff. Water pooling at the base of a wall is a classic. Third, nearby works: an excavation next door, earthworks that destabilise.

On an old house, materials are often soft (limestone, lime mortars). They cope less well with movement. A stair-step crack crossing several stone courses generally indicates differential movement, not simple crazing. Crazing is superficial: a fine network, no offset. If you run your hand over it and feel a step, you’re in a different category.

How to assess severity without fooling yourself

Three criteria matter. Width: below 0.2 mm, it’s often cosmetic. Between 0.2 and 2 mm, monitor it. Above 2 mm, especially if it’s evolving, you’re in structural territory. Continuity: a crack running from the ground up to the roof across floors is more serious than a local crack under a window. Offset: if the two edges are no longer in the same plane, the movement is real.

Honestly, the only opinion that counts is from a professional who comes to see it. A facade diagnosis is an hour on site, a survey, and a clear conclusion. Don’t settle for a remote patching quote.

What to do in practice

1. Set a tell-tale and photograph it with a ruler for scale. Date everything. 2. Check rainwater drainage: gutters, downpipes, inspection chambers. A blocked downpipe can be enough to explain a disorder. 3. Don’t patch immediately. Patching an active crack hides the problem, it doesn’t treat it. 4. If the crack evolves or exceeds 2 mm, call in a building expert or a structural engineering firm. 5. If a neighbour has done recent works, document the timeline.

Recourse and guarantees

If the house is under ten years old, the ten-year guarantee may apply (Article 1792 of the French Civil Code). For an old house, it’s a different regime. You can pursue the company that carried out recent works if the disorder is attributable to them, on the basis of contractual liability. In a dispute with a neighbour over works that destabilised your wall, Article 1244 of the Civil Code (formerly 1386) may be invoked. And if the disorder stems from a recognised natural hazard, natural disaster insurance may apply after an interministerial decree. In Nice, decrees for ground movement are not rare. Keep all evidence: dated photos, tell-tales, invoices.

Frequently asked questions

Is a stair-step crack always serious?

No. A fine, stabilised crack can remain cosmetic for years. What matters is its evolution and width. Above 2 mm with offset, you should consult.

How do I know if my crack is still moving?

Place a tell-tale (plaster or pencil line) across it, date it, and check every three months. If the tell-tale breaks, the crack is active. A crack gauge gives a more precise measurement.

Can I patch it myself?

Patching an active crack without treating the cause is a bad idea: it will reappear. Wait for the diagnosis. If it’s stabilised and superficial, a suitable flexible mortar may suffice.

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